Biocompatibility is a critical concern for medical devices that are designed to be implanted in vivo. Biocompatibility is necessary to avoid adverse reactions in the subject, and to avoid device failure as a result of exposure to the corrosive saline body fluids and other substances in the tissue surrounding the implant. Where an implanted device includes one or more components that are not, themselves, biocompatible, it is known to provide hermetic sealing of such devices with a chemically inert coating to achieve biocompatibility, i.e., in order to avoid adverse reactions and device degradation. Many such implantable devices are intended to remain in place over long periods of time, imposing a long life requirement on the hermetic sealing.
Large implantable electronic devices, such as pacemakers, are typically enclosed within a hermetic case. Size and thickness is not critical to such devices and so it is relatively easy to provide hermetic encasement. However, micro-miniature implantable devices, which commonly include microelectronic components such as integrated circuit chips fabricated on silicon substrates, are generally not encased and instead, use relatively thin layers of a deposited hermetic material for sealing. Such micro-miniature devices include, for example, implantable nerve stimulators such as visual prostheses, cochlear prostheses, deep brain stimulators, spinal chord stimulators, and functional electrical stimulators for motor control. In the case of micro-miniature implantable medical devices, biocompatible and electrically insulating metal oxide films have been deposited on the surface of components, such as integrated circuits, passive electronic devices and components, magnets, and mechanical pieces, in order to passivate them and make them less susceptible to attack in the body. These are referred to as “hermetic coatings”, where the word hermetic is used to mean that the films do not leak significantly, and thus prevent fluids, and materials in the fluids, from reaching the components to be protected.
Ion beam assisted deposition (“IBAD”) of ceramic materials such as alumina, (often referred to an aluminum oxide (Al2O3)), has been used for hermetically sealing micro-miniature devices. Alumina has good biocompatibility, and IBAD is a useful technique for depositing dense, adherent, defect-free conformal thin films. The use of IBAD to deposit alumina on implantable medical devices is described in U.S. Pat. No. 6,844,023, entitled “Alumina Insulation For Coating Implantable Components And Other Microminiature Devices,” the disclosure of which is incorporated by reference. IBAD may be used to deposit electrical insulators on integrated circuits, passive components, magnets, and other implantable devices in order to provide a thin, hermetically sealed package. Typically, layers deposited using IBAD are only a few microns thick.
The inventors have found that a thin insulating layer, such as alumina deposited by IBAD, may not provide adequate long-term protection of implantable devices. Specifically, the inventors have determined that such layers are subject to erosion in the body and are, therefore, susceptible to failure over time. Some metal oxides, such as alumina, although generally considered to be inert, undergo slow reactions in the presence of water. Whether the reactions are due to aqueous chemistry, the presence of ions in solution, the presence of atomic oxygen, or some other mechanisms or combinations of mechanisms, the resulting reactions produce changes in the surface of the metal oxide and a slow, persistent thinning of the insulating layer. Where the insulating is a thin film that was deposited to protect a device, the resulting erosion of this film compromises the functional utility of the film. Thus, the utility of known hermetic coatings for micro-miniature devices is jeopardized by the reactive processes that eventually result in the failure of the thin insulating coating.
In order to use thin hermetic insulating films as protective coatings for micro-miniature implantable devices intended for long-term use, a way of extending their lifetime is required. One possible way to extend the lifetime of the insulating coating is to increase the thickness of the deposited film. However, this approach is often incompatible with the intrinsic stress of the film which tends to build with increasing thickness, ultimately causing the film to crack. Even if the problem of stress could be overcome, the slow growth rate of such films (e.g., 1-2 Angstrom/sec), makes the growth of thick films unattractive from a manufacturing perspective, and so an alternative to merely increasing the thickness of the insulating layer is desirable.